Rolf Harris did not have anything to prove when he moved from the dock to the witness box, a judge has told a jury.

It has always been up to the prosecution to make the jury sure of his guilt.

On Tuesday (overnight NZT) Justice Nigel Sweeney began summing up the law and evidence to the jury in the Australian entertainer's sex assault trial.

Harris has pleaded not guilty to 12 counts of indecently assaulting four girls between 1968 and 1986.

The jury will be asked for a unanimous verdict on each count.

Justice Sweeney told the jury to use their experience to "see the wood for the trees and recognise a red herring when you see one".

He warned them not to read anything into the fact that Harris moved from the dock to the witness box to give evidence.

There was no burden on Harris "to prove anything at all", he said. "The prosecution must make you feel sure of guilt."

To be sure of sexual assault they must be sure that Harris touched the complainants in an indecent manner, and without their consent if they were not under 16 at the time.

"Consent obtained through compulsion is not true consent," he said.

Justice Sweeney called for a "cool, calm, careful and dispassionate" consideration of the evidence, saying they should have the "courage to return true verdicts based on the evidence whatever the consequences may be".

He said the prosecution case in a nutshell was that the accounts of the complainants and other witnesses were "tellingly similar" and there was no reason that any, let alone all of them would have lied.

The defence case "relies strongly on [Harris'] good character", and was that his accusers had lied - all except one, who may have been mistaken as to the identity of her attacker.

The judge went through the "potential similarities" that the prosecution had identified between the allegations, and in the stories of seven ''bad character witnesses''.

"The prosecution suggest that that is no coincidence," he said. "The fact that seven witnesses made similar but unconnected [save for a mother and daughter pair] claims makes it more likely that the allegations made by the complainants on the indictment are true ... you are perfectly entitled to view it in that way."

The more similar the stories, the less likely that the similarity could be "explained away".

However it was up to the jury to decide what to make of the similarity in deciding their verdicts.

On Monday the defence argued that prosecutors had "trashed" Harris' reputation, but fell a long way short of proving him guilty of sexual assault.

Defence counsel Simon Ray said the Crown had tried to paper over the weaknesses in its case against Harris with aggression and name-calling.

But Ray said Harris, despite his shame and embarrassment over an adult affair with one of the complainants, should be believed when he denied assaulting any of his accusers.

Prosecutor Sasha Wass, QC, told the jury last week Harris was a "sinister pervert who had a demon lurking beneath the charming exterior".

Wass said the prosecution's evidence was like one of Harris' own paintings: "Each stroke can be a little bit vague or unclear but taken together you can identify what is happening."

"The evidence paints a picture of a man ... arrogant, brazen, who treated women and young girls as sexual objects to be groped and mauled whenever he felt like it," Wass said.

The main complainant, a childhood friend of his daughter Bindi, "was targeted, groomed and dehumanised by Rolf Harris over a period of 16 years," she said.

Wass compared the evidence from the main complainant with that of the three others - including Australian Tonya Lee - and six character witnesses who had allegedly witnessed or experienced assault at the hands of Harris.

Unknown to each other, from "one end of the world to the other" they had described a similar pattern of "deviant behaviour", Wass said - the idea that they had all invented such similar accounts was "absurd".

Justice Sweeney was to finish summing up at Southwark Crown Court on Wednesday (Thursday), and the jury are likely retire to consider their verdict that day.